This invention relates to a method and apparatus for reforming the base of a container. In particular, but not exclusively, it relates to a method and apparatus for reforming the base profile of a can having a one-piece can body used for packaging carbonated beverages.
Such so-called "beverage cans" typically comprise a side wall, a transition region and a base including a stand bead, an annular inner wall and a substantially dome-shaped centre panel. When such beverage cans are manufactured on a bodymaker, the base formed by the dome station usually has a radially inwardly tapering inner wall for ease of removal from the domer. However, in recent years it has become common practice to reshape this inner wall at least to a vertical profile or, more usefully, to a negative angle, or a hooked or beaded profile.
The benefits of reforming the base profile have been demonstrated in terms of limiting dome growth and increasing dome reversal pressure. When the can is pressurised, the dome will "grow" outwards, pushing the metal around the stand diameter so that the stand bead will roll out and the can height will increase. Dome growth is thus a particular problem when the can needs to withstand pasteurisation pressures, such as is the case when the product is beer, for example. Dome reversal pressure is the pressure at which internal pressure from a carbonated beverage, for example, will cause the dome-shaped centre panel to reverse its shape completely from externally concave to externally convex.
Although the principles of base profile reforming are well-known, apparatus which has been available in order to carry out the reforming process has not been found to be entirely satisfactory. External base profile reforming which is carried out by applying radially inward pressure to the transition region of the can is well-established but has problems in terms of defining the final shape which is achieved and/or tool removal where a negative angle is formed by pressing the inner annular wall onto a shaped chuck.
Internal base profile reforming involves the direct application of a roll against the inner wall so as to reform part or all of that wall to a specific new profile. One particular problem which has been found with known internal base profile reforming tooling is that there is a very high rate of wear of tool parts. This wear is particularly costly in terms of replacement parts and, if components are not replaced, will lead to variability in the very profile which is critical to improving can performance.
This invention seeks to provide a solution to these problems, particularly for internal base profile reforming, by eliminating wear as far as possible.